This invention relates broadly to the removal of boron from brines, and more particularly to an improved process for removing boron from magnesium halide-containing brines.
In the preparation of magnesia, MgO, it is well known to calcine the mineral magnesite, MgCO.sub.3, a basic magnesium carbonate, or the naturally occurring mineral brucite, Mg(OH).sub.2, at temperatures above about 2700.degree. F, particularly at from about 3000.degree. F to about 4000.degree. F. Alternately, magnesia can be prepared by calcining magnesium hydroxide, MG(OH).sub.2, obtained from magnesium halide-containing brines, such as sea water, by reacting them with dolime. The reaction produces a solid magnesium hydroxide, which, after being washed to remove chlorides and filtered to remove the liquid by-product, is passed through a kiln at calcining temperatures for conversion to MgO.
The characteristics of the magnesia products thus obtained can be varied by modification of the processing parameters, such as the kiln temperature, in accordance with the desired end use of the magnesia. A commonly desired magnesia product is periclase, a dead burned, well-crystallized material used in the manufacture of refractory brick. It has been found, however, that the impurity boron, probably present in the form of borates, causes processing difficulties in the production of certain forms of refractory brick and remains as an undesired impurity in the finished product. As a result, it is highly desirable in the art to have a relatively low boron-containing MgO product, i.e., one having a boron content of less than 150 parts per million (ppm) boron on an oxide basis. For this purpose, magnesium hydroxide product obtained from magnesium halide-containing brines must be one having such a low boron content, i.e., below 150 ppm (parts per million) and preferbly below 75 ppm. The boron content of the magnesium hydroxide can be controlled to such low levels either by careful control of the brine-dolime reaction or by removing or significantly reducing the boron level in the magnesium halide-containing brine prior to reaction with dolime.
While the boron content of brines will, of course, vary considerably from one source to another, a typical brine employed in refractory production operations in Michigan, containing about 95-115 gpl (gram per liter) MgCl.sub.2, has a boron content of approximately 50 mg/l (milligrams per liter), i.e., 40 ppm at a specific gravity of about 1.24 - 1.26. If such a brine is employed in excess of the stoichiometric requirements of the brine-dolime reaction, most of the boron from the brine will be found, by analysis, in the Mg(OH).sub.2 product. When using a brine/dolime weight ratio of 12/1 with the above-mentioned brine, the amount of boron found in the Mg(OH).sub.2 product is 480 ppm, which is equivalent to about 700 ppm boron on an oxide basis.
In order to obtain a relatively low boron-containing MgO, i.e., less than 150 ppm, it is necessary to regulate the processing conditions during the reaction of brine and dolime to operate on the highly alkaline side to obtain Mg(OH).sub.2 containing less than 150 ppm boron on an oxide basis. In a continuous system, an upset in the process can cause the production of off-specification Mg(OH).sub.2 having more than 150 ppm boron, with the off-spec material becoming a part of the overall system, rendering all of the Mg(OH).sub.2 in the system out of specification. One of the major ways of overcoming such off-spec Mg(OH).sub.2 is by dilution. A considerable amount of time and Mg(OH).sub.2 be be lost, in such circumstances, due to an upset in the system when producing a low boron product.
A method of assuring a low boron product without such processing difficulties would be to remove the boron from the brine before reacting said brine with dolime. A number of techniques for pre-treatment of magnesium halide-containing brines for this purpose have been proposed. In such techniques, particularly where CaO or dolime are employed in the pre-treatment of brine, the MgCl.sub.2 content of the brine is reduced. Such reduction of brine strength is undesired with respect to the subsequent use of the brine, as by reaction thereof with dolime. Still further, the methods for producing magnesium hydroxide containing less than 150 ppm (oxide basis) boron frequently results in a product contaminated with 1.5 - 2.0% CaO (MgO basis) which is introduced in the refining process and which is a commercially unacceptable product since not more than 1.0% CaO is preferred, although up to 1.5% CaO is acceptable.
In the production of magnesia and related activities, there is, as a result, a continuous desire for improvement in the techniques available for production of low boron products from magnesium halide-containing brines, especially those products which are low, i.e., less than 1.5 percent, in CaO as well. In particular, there is a need for new techniques for pre-treating brines to remove boron therefrom prior to the reaction to the brine with dolime.